The grid stopper and g2 resistors have practically no effect to THD.
But the used anode load, cathode resistor and supply voltage have a lot.
And you do not mention about these essential things at all ??
How did you ensure that you used optimum conditions with 6E5P.
What were your THD values with 6E5P ?

No THD measured, but.

-1.2V fixed battery bias
28mA Ia
~110 Ua
8k resistor load
330V supply

It seems to me in pretty linear zone judging from the triode strapped graphic.

According to my knowledge your operation conditions are very far from optimum.
The first and absolute requirement is to use CCS loading. Otherwise optimum linearity and high output voltage can not be achieved.

When high output level is required the grid voltage must be relatively high.
With -1,2 V the grid current begins to flow almost at once.

In whole, this is very typical situation among such hobbyists who only evaluate circuits by listening.
You had bad operating condition but the reason for bad sound is entitled to the used tube only.

Some tubes can work better with such bias that some other tubes.
When bias is -1,2 V only and input signal is 1 Vrms, then this gives some 2,8 Vpp grid voltage and grid goes positive, grid current flows and tube begins to distort.

if the gain is ~ 30 then for 180v drive you'll need 180/30 =6v p-p into the tube and that means driving it 1.8v positive. I dont think the 6e5p is great A2, i think the 6J9p is good A2 which accounts for its voltage swing. most designs limit themselves to -1 so 1.2 bias gives about 12v before grid currant is likly to flow.

I do need to actually listen to these tubes and alot of them have similar pinouts other than pin8/9, the 6e5p is compleatly different though.

i have 6J52 6P15p-ev and 6P1P on there way for tests.
I did measure the 6s45 but my signal generator at the time was too limp to give good results. tested at bias points between 1.25 to 3v in .25 volts and found 0.031 at 22ma -1.75/160 this is before i added variable currant, will give it another go for big voltage swings.
p.s i'm ignoring alot of max specs here as lifetime on a 1-2£$ tube will not keep me up at night!

these tests had a 600r resistor in the grid tag to subdue the oscilation that kicked in as i raised the currant
Best results were all at 28ma

-1.08 155v 28ma 0.15% 61vrms/170p-p

and tried for max swing before clipping, i'm sure you could get more but i'm using a switched attenuator so adjustment is coarse.

-1.42 171v 28ma 0.29% 85vrms/238 p-p

So the 6c45 is up with the best but needs running heavily in A2 to do it 1.5vrms/4.2p-p in at 1.1v bias so 1v+ on the grid. This was driven by my 4P1L preamp Z out~1-2K and thru the 600r resistor so doesnt need to source much currant it seems.

According to my knowledge your operation conditions are very far from optimum.
The first and absolute requirement is to use CCS loading. Otherwise optimum linearity and high output voltage can not be achieved.

When high output level is required the grid voltage must be relatively high.
With -1,2 V the grid current begins to flow almost at once.

In whole, this is very typical situation among such hobbyists who only evaluate circuits by listening.
You had bad operating condition but the reason for bad sound is entitled to the used tube only.

Completely agree. For a driver why not implementing a gyrator in mu-follower output instead of an CCS? The gyrator in mu follower will provide the lowest output impedance and sufficient current in particular if you are planning to drive the valve in A2.

I'm currently refining my driver for the 4-65a SE amp which demands 200Vpp at the grid in A2. I'm testing a 46 (triode-strapped) in filament bias with the gyrator. Yes, a waste of heat, but the 46 sounds brilliant in my experience. The driving results are very good at maximum drive: 0.18% THD @ 200Vpp

4P1L is definatly on the cards for me but will involve more work to test and finally impliment in the amplifier, two more tx and reg boards to fit into a two chassis heavy, hot amplifier.

p.s re: 6c45pi.. the minimum thd was with 2nd and 3rd harmonics equal, if the currant was dropped the 2nd and hence thd went up, but the spectrum improved with a lower 3rd and the harmonics over 4th went into the noise floor. so perhaps 22-3ma would be a better comprimise at ~.2%

Enzo,
The measures on 6c45 look really good.
Why not building a mu-follower gyrator load for the 6C45 driver? You can then drive the output valve in A2 easily
Cheers,
Ale
PS: where are you located in the UK?

I'm near farnham surrey, I'm actually using a mu follower with a triode connected 6e5p, 900v ht so i'm not short of volts and i dont plan any excursions into A2, at least not in the GM70's! sounds nice to me.

My opt are rated at 14w at 11k5 so the 15.5 its making at 5% is probably my lot for now, but i'd like 15w at <5%

Im not planning on using the 6c45, but sticking with the 6e5p's, 0.09% at 200v made my jaw drop open! and keeps things single stage. Though these tests have made me realise how precise the bias needs to be at these outputs. might go with some regulated fixed bias supplies on drivers as well as output valves.

I'm using a 337 negative regulator for the -90v bias on the outputs for now, just one supply to both gm70's derived from an 18v secondary with voltage multiplier, so plan to build individual regulators, probably a better raw power supply too. maybe a shunt reg with a series zenner chain like i believe the MJ statistical reg and a rotary switch to adjust the bias point....

I've not had a chance to hear any of these valves, other than the 6e5p, on the gm70 yet so these are just figures, but listening tests will be fun:-)